With the recent improvement of a wireless communication infrastructure, wireless data communication with a portable electronic apparatus, such as a notebook computer (hereinafter, referred to as ‘note PC’), outdoors is increasingly demanded.
In addition, under the infrastructure for wireless communication having different coverage or throughput, when wireless data communication is performed with the note PC outdoors, a plurality of wireless communication terminals having different careers may be connected to a single PC.
For example, when the note PC is used outdoors, power supply depends on a battery of the PC.
In recent years, there is known a note PC whose continuous operation time is 4 hours or 7 hours. In this case, the continuous time to be stated in this specification is based on the JEITA measurement method 1.0.
However, this measurement method gets a rough standard of the standalone operation time of the note PC, and measurement is performed in a state where no external device is connected thereto. Accordingly, the measurement method is not partially suitably used under the recent environment.
Recently, in regards to the external device, unlike a known low power consumption device, such as a USB mouse, an external storage device, such as a USB (Universal Serial Bus) memory or a CD-RW, a DVD-RW, or an HDD (Hard Disc Drive) has been used outdoors.
In case of such an external device, power supply depends on the battery of the PC.
As the wireless communication terminal that is connected to the note PC, there is known a wireless communication terminal that uses a wireless communication system corresponding to high data communication speed.
As the high-speed wireless communication terminal, a wireless LAN, which is used indoors, for example, an office, and a cellular system (CDMA2000 or W-CDMA), which is used over a wide area, may be exemplified.
The wide-area wireless communication terminal requires a baseband processor, such as a high-speed CPU (Central Processing Unit), or maximum transmission power of 20 dBm (100 mW) or more, which causes an increase of power consumption. For example, in case of CDMA2000 1× (and CDMA2000 1×EVDO), power consumption during maximum output transmission may be around 2.8 W.
The external device can be supplied with power from the note PC. In this case, power to be supplied is defined according to the specification. For example, a PCMCIA (Personal Computer Memory Card International Association) connector is 5 V 600 mA, Cardbus is 3.3 V 1 A, and a USB device is 5 V 500 mA.
However, these specifications may not take unexpected power consumption or the battery of the note PC into consideration.
In addition, among the note PCs, there are known the note PCs that use a method of taking precedence the operation time in the battery and setting a voltage to be supplied to an external interface (I/F) to a minimum normal value, or in which a supply current is lowered less than a normal value due to insufficient current supply ability when a large current flows.
Even if the external device is connected in a state where power is supplied, in view of convenience, the external device may be removed from the note PC, which is called ‘hot plug’. Accordingly, an I/F between the external device and the note PC can be easily connected and disconnected, and thus the external device is not connected to the note PC until being used, which contributes to low power consumption.
Furthermore, as a method of reducing power consumption in the note PC, for example, there is generally used a method that changes an operation clock or a voltage according to the load condition of the CPU, or a method that halves luminance of a backlight of an LCD (Liquid Crystal Display) according to the connection state of a power supply, or takes a pause or lowers power according to set time or the condition of an internal battery.
Patent Document 1 discloses a method that supplies power from the outside to a device, to which insufficient power is supplied, separately from current supply from the external device connection terminal.
For the power control of an external device, Patent Document 2 discloses a method that, when a plurality of externally connected devices are connected together to the USB, and respectively consume power, supplies a limited amount of power based on the USB standard.
In regards to a cellular phone having an internal power supply, there is known a technology that, when a battery voltage is lowered, controls a transmission output such that the set value of the maximum transmission output is less than that during the normal operation, thereby suppressing transmission power.                Patent Document 1: JP-A-2000-10676        Patent Document 2: JP-A-2003-280775        